G-protein coupled receptor systems are ubiquitous signal transduction systems found in many higher organisms. Extracellular signals are transmitted across the cell membrane via a seven transmembrane receptor protein which activates G-proteins, initiating a complex biochemical cascade of events. This activation process is down regulated by a growing family of proteins called arrestins which bind to the activated receptor and prevent G-protein activation. Arrestins appear to serve as adaptor molecules for receptor internalization from the plasma membrane. We plan to collect a complete native data set as well as potential heavy atom derivatives.